1. Field of the Invention
The invention relates generally to the field of mobile trackways. More particularly, it relates to mobile trackways adapted to move over existing trackways, for example those found in tunnels.
2. Description of the Prior Art
Railroad type trackways are, as a rule, laid in the tunnel behind advancing tunneling equipment so that muck cars be brought to the tunnel head for removing the tunnel borings (commonly called muck). Ordinarily, because of the expense, and for other reasons, only a single trackway is laid. Consequently, when one string of muck cars has been loaded at the tunnel head, the tunneling operation must be curtailed until that string of cars can be removed and an empty string can be moved into loading position. Since many tunnels are several miles long an appreciable amount of time is thereby wasted and tunneling costs are correspondingly increased.
To prevent such tunneling interruptions, and as an alternative to laying a second trackway in the tunnel (which may be additionally difficult in a round tunnel because of tunnel curvature and which requires two separate locomotives and/or crews, with consequent great added expense), comparatively short, mobile or movable dual trackways are used. These movable dual trackways are advanced behind the tunneling equipment and provide for two side-by-side strings of muck cars at the tunnel head where they are needed. Empty muck cars on one of the trackways may be loaded while loaded muck cars are removed from the other trackway. Muck cars are brought to, and removed from, the movable trackway via a single trackway laid on the tunnel floor.
Heretofore available movable dual trackways have, however, frequently proved impractical or impossible to use, particularly in round (in cross-section) tunnels having sections which are unlined.
Relatively wide, flat tunnel floors are required to accomodate some types of movable, unwheeled dual trackways which are slid along the tunnel floor on skids in advance of a conventional trackway. These types generally comprise several interconnected sections which are advanced one section at a time in "inchworm" fashion. Tunneling equipment must often be placed on sections not being moved to anchor (by friction) these sections to the tunnel floor while another section is pushed or pulled (as by hydraulic pistons) forwardly. Although the expense of laying a second trackway in the tunnel is thereby avoided, a considerable amount of time is wasted in inching the movable trackway forwardly and in moving the tunneling equipment about. It is emphasized that when the tunneling equipment is being used to weight sections of the movable trackway, it cannot be used to tunnel.
Lined tunnel walls, having affixed thereto special guide members or rails, are required for the movement of other types of movable dual trackways. These movable trackways are elevated above a tunnel trackway and are slid along the guide members. Although they are more adaptable for round tunnels than the previously described types, they require tunnel liners to support their weight and the weight of the loaded muck cars. Many sections of tunnels, however, otherwise require no lining, hence use of these types of movable trackways requires expenditure of considerable extra time and money. It is economically impractical to line tunnel portions, which portions would otherwise not need lining, simply to accommodate movable trackways.
Ideally, movable dual trackways should have wheels and be transportable along the existing tunnel trackway in the manner of some existing, movable single trackway units. But eccentric forces caused by the loaded muck cars on one of the dual trackways, while the other trackway is empty of cars, tend to cause the movable trackway to tip over sideways. It has heretofore been necessary to employ outriggers or braces which bear against the tunnel to prevent this tipping. However, with such braces in position it is virtually impossible to move the trackway. The trackway must thus remain stationary as the muck cars thereupon are loaded, even though the tunneling equipment itself is advancing. This requires additional time and expense to move the muck from the tunnel head to the cars. The problems involved in positioning and removing braces are major and time-wasting.
Existing movable single trackways which are adapted to roll along another trackway do not encounter the problem of eccentric loading because the single trackway is mounted along the center line of the unit. Thus there is little tendency -- other than as a result of a high center of gravity -- for such trackways to tip over. But movable single trackways are generally limited to the transport of, for example, wider gauge cars over a narrow gauge trackway, and are obviously not adaptable for the purpose of transporting two side-by-side strings of load-bearing cars.
Heretofore, to the applicant's knowledge, there have been no movable dual trackways adapted to roll over existing tunnel trackways which do not require such bracing as described above and which may be moved in substantial unison with the tunneling equipment.